feature: add AddTrailingSlash

Author: duhnnie

byteseq.go

@@ -87,3 +87,30 @@ func TrimRight[S byteSeq](s S, cutset byte) S {
 	}
 	return s[:lenStr]
 }
+
+// AddTrailingSlash appends a trailing '/' to v if it does not already end with one.
+//
+// For string inputs, the result is always a new string.
+//
+// For []byte inputs, the result is always a new slice with the trailing '/' appended.
+// The original slice is never modified, even if it has sufficient capacity.
+//
+// This function guarantees that the returned value is independent of the input.
+func AddTrailingSlash[T byteSeq](v T) T {
+	if len(v) > 0 && v[len(v)-1] == '/' {
+		return v
+	}
+
+	switch x := any(v).(type) {
+	case string:
+		return any(x + "/").(T) //nolint:forcetypeassert,errcheck // can't fail
+	case []byte:
+		result := make([]byte, len(x)+1)
+		copy(result, x)
+		result[len(x)] = '/'
+		return any(result).(T) //nolint:forcetypeassert,errcheck // can't fail
+	default:
+		// impossible because of the constraint
+		return v
+	}
+}

byteseq_test.go

@@ -303,3 +303,76 @@ func Test_Trim_Edge(t *testing.T) {
 		})
 	}
 }
+
+func Test_AddTrailingSlash_String(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		in   string
+		want string
+	}{
+		{"", "/"},
+		{"abc", "abc/"},
+		{"abc/", "abc/"},
+		{"/", "/"},
+	}
+
+	for _, tt := range tests {
+		require.Equal(t, tt.want, AddTrailingSlash(tt.in))
+	}
+}
+
+func Test_AddTrailingSlash_Bytes(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		in   []byte
+		want []byte
+	}{
+		{[]byte(""), []byte("/")},
+		{[]byte("abc"), []byte("abc/")},
+		{[]byte("abc/"), []byte("abc/")},
+		{[]byte("/"), []byte("/")},
+	}
+
+	for _, tt := range tests {
+		require.Equal(t, tt.want, AddTrailingSlash(tt.in))
+	}
+}
+
+func Benchmark_AddTrailingSlash(b *testing.B) {
+	tests := []struct {
+		name string
+		in   any
+	}{
+		{"StringSmallNoSlash", "abc"},
+		{"StringSmallWithSlash", "abc/"},
+		{"StringLargeNoSlash", string(make([]byte, 10_000))},
+		{"StringLargeWithSlash", string(append(make([]byte, 10_000), '/'))},
+
+		{"BytesSmallNoSlash", []byte("abc")},
+		{"BytesSmallWithSlash", []byte("abc/")},
+		{"BytesLargeNoSlash", make([]byte, 10_000)},
+		{"BytesLargeWithSlash", append(make([]byte, 10_000), '/')},
+	}
+
+	for _, tt := range tests {
+		b.Run(tt.name, func(b *testing.B) {
+			switch v := tt.in.(type) {
+			case string:
+				var out string
+				for i := 0; i < b.N; i++ {
+					out = AddTrailingSlash(v)
+					_ = out
+				}
+
+			case []byte:
+				var out []byte
+				for i := 0; i < b.N; i++ {
+					out = AddTrailingSlash(v)
+					_ = out
+				}
+			}
+		})
+	}
+}